Field of the Invention
The present invention concerns a system for the calculation of the weight of a load that is manipulated with the aid of lifting equipment. The invention concerns also a load-sensitive jointed connector between the end of a crane arm and a cross-piece that is jointed at the same and that allows the load to rotate or to be tilted around an axis of rotation relative to the crane arm.
Description of the Related Art
It is in many cases desirable to obtain knowledge of the weight of a load that is manipulated by lifting equipment such as a crane or similar, and in this way the weight of the total amount of goods manipulated. This type of information is important in order to make it possible to use the full loading capacity of a vehicle while at the same time avoiding overload. It is not possible to obtain the said information if the weight of the goods is not known in advance. It is also necessary in many cases to know how much load a crane arm manipulates during a certain period, not least for statistical purposes, and in order to make it possible to carry out calculations that are necessary for control and follow up. Timber, wood for pulping and wood for energy, are typical examples of products whose weight is difficult to determine without weighing. If load-sensitive arrangements are not available, the risk always arises that loading will take place without the required control, and with the associated risk for overload or for inefficient use of the required loading capacity of a transport vehicle.
Vehicle-mounted cranes that are used during the loading of forestry products are generally provided with a gripper for lifting and manipulating loads of timber. The gripper is suspended at the crane arm through a joint, in a manner that allows pivoting. Rotation of the gripper takes place by means of what is known as a “rotator”, whose turning torque is transferred to the crane arm through a jointed connector in the form of a rigid connecting link that normally has a cardan design, in order to allow the gripper that is suspended at the crane arm to pivot in the manner of a wrist joint around two perpendicular principally horizontal axes. Rotation of the gripper around an essentially vertical axis normally takes place by a hydraulic motor. Combination machines are also known in which the gripper is replaced by a harvester head that can be tilted into various operating positions relative to the crane arm. Tree trunks are manipulated where relevant also by a combination of crane arm and timber gripper that have been mounted on a timber lorry.
Lifting equipment of the type specified above with integrated load cells and that can give, in response to an applied load, a signal that corresponds to the force that the applied load exerts on the load cell is previously known. Among these, the following can be mentioned:
U.S. Pat. No. 3,911,737 concerns an arrangement for the weighing of loads that have been lifted by a gripper, such as an individual tree trunk or a bundle of trunks. A load cell is connected in the arrangement between the end of a crane arm that is a component of the arrangement and a rotator, in order to measure the lifting force that arises. The load cell is designed as a suspension scale that extends over the distance between the free end of the crane arm and the load that is to be carried. The extra distance between the crane arm and the load that the suspension scale introduces results in an increased risk of oscillations and it complicates the manipulation of the load. The unprotected design involves a high risk of damage to the load cell from impacts and collisions.
An arrangement is known from SE 527,169 C2 for the calculation of the weight of a load that is manipulated with the aid of lifting equipment that includes a lifting boom intended to support a load through an intermediate link. The intermediate link allows the load to oscillate in a cardan manner around horizontal mutually perpendicular axes. The load cell in this design is integrated in a discrete manner in an end part of the forward end of the lifting boom, whereby the end part is provided on its outer surface with an indentation, in which a load cell and its associated strain gauge are intended to be mounted.
A load-sensitive arrangement is known from FI 84759 C intended to be used at a crane arm provided with a timber gripper. A connection that is located between the end of the crane arm and the timber gripper includes a cross-piece and a rotator. The cross-piece is provided on its outer surface with grooved depressions in which load-sensitive strain gauges are mounted. The strain gauges measure the deformation that arises from the load, and generate a signal that is proportional to the load.
During the manipulation of load, the crane arm is subject to uncontrolled static and dynamic effects of forces. The effects of these loads increase in the direction in towards the jointed attachment point of the crane arm at a base machine that may be constituted by, for example, the wheeled chassis of a forestry machine. The said external forces and effects have a detrimental effect on the measurement results from the load cell, and must be filtered out in order for as correct a measurement result as possible concerning the weight of the load to be obtained. In other words, the further away the load cell is located from the load, the more difficult it will be to calculate the weight of the load accurately. At the same time as the location of the strain gauges of the load cell as close as possible to the load makes it possible to obtain more accurate measurement results, there arises not only the above-mentioned problem of the risk of damage to the load cell due to its exposed position close to the load, but also problems with efficiently and securely supplying the load cell with power and of leading away output signals from the load cell to some type of calculation means or weighing instrument in order to determine the weight of the load. The said calculation means is normally located at a distance away from the load cell, for example on the base machine that supports the crane arm. It should be understood also that in the same way as a load cell that is located relatively close to the load risks being exposed to damage and impacts, also the sensor cable that supplies the measurement cell with power and that leads an output signal from the measurement cell runs a great risk of being damaged, of becoming stuck in an object and becoming crushed during manipulation of the load by means of the crane arm.